Aluminum borate whisker (Al₁₈B₄O_33w, denoted by ABO_w) reinforced AZ91D (Mg-Al-Zn) magnesium alloy composites were prepared by squeeze casting. The composite was solution-treated at 683 K for 345.6 ks in argon atmosphere and then water-quenched. Aging treatment was carried out at 443 K in oil-bath. While the composite was cooled from the solution temperature of 683 K to room temperature, large thermal residual stress (TRS) was formed due to the mismatch of thermal expansion coefficient between ABO_w and AZ91D matrix. The TRS of AZ91D matrix in as-quenched AZ91D/ABO_w composite was measured with X-ray diffraction (XRD) and it was 103 MPa. After aging for 115.2 ks, the TRS relaxed and decreased to 45 MPa. Owing to the formation of precipitates, the bending strength of peak-aged composite (aged for 115.2 ks) was enhanced. Nevertheless, when the composite was aged for 345.6 ks, the bending strength decreased greatly. Even the bending strength of over-aged composite was lower than that of as-quenched composite. It is attribute to the coarsening of continuous precipitate within composite matrix close to whiskers.